1. Field of the Invention
The invention relates to an optical pick-up head with a single objective lens and, in particular, to an optical retrieving apparatus that retrieves data from CD (compact disc) and DVD (digital versatile disc) of different thickness using a single objective lens.
2. Related Art
The technologies of storing data on CD and DVD and accessing data stored on these two types of discs via an optical retrieving apparatus are well known. The access to the data is proceeded via the optical pick-up head. The principle of retrieving data from the discs is: first focus a laser beam into a spot on the data storage surface of the disc, then convert the data signal beam reflected from the surface of the disc into recognizable electrical signals with the help of a photo detector. The process of storing data is the reverse of retrieving. The size and specification of the CD and DVD are listed in Table 1.
TABLE 1 THICKNESS TRACK PITCH WAVELENGTH mm .mu.m nm NA CD 1.2 1.6 780 0.45 DVD 0.6*2 0.74 635-650 0.6 NA: numerical aperture
From Table 1 one can obviously see that DVD has a relatively higher storage density than CD. The differences in both sizes and, in particular, storage densities result in different requirements for the optical accessing apparatus. To retrieving data on the data storage surfaces of CD and DVD, the optical pick-up head plays an important role, whereas the optical system design is the most important part in the optical pick-up head.
Therefore, one needs to design an optical system in which the laser beam focuses on the correct spot with a proper size using two different numerical apertures to have a correct access the data on CD and DVD according to the respective thickness of the discs.
Owing to the compatibility issues, commercialized products using a single optical pick-up head have to be able to correctly access the data on the CD and DVD, and, therefore, how to make a dual focusing optical system is the key point.
The technologies of making dual focusing optical pick-up head for accessing data on CD and DVD can be classified into the following categories:
1. Use two objective lenses with different foci in the system, and then select one of them by a driving mechanism according to the type of the discs (CD or DVD). The drawback, however, is that both the weight of the optical pick-up head and its price increase significantly. PA1 2. Put diffractive element on the objective lens to achieve the dual focusing effect. PA1 3. Use holographic optical element (HOE) to generate double foci. Nevertheless, it is hard to manufacture and requires a higher cost. PA1 4. Make a numerical aperture controller with the liquid crystal display (LCD) shutter so that the beam passes through the objective lens with two different numerical apertures to achieve the dual focusing effect. But the polarization of the laser source has to be accurately controlled so as to cooperate with the LCD shutter. In addition, the operation of the LCD shutter demands a continuous power supply. PA1 5. Make a ring structure on the objective lens. Retrieve the CD data by the beam passing through the central area while access the DVD data by the combination of the beams passing through the inner circle and the outer ring. For example, the U.S. Pat. No. 5,665,957 (illustrated in FIGS. 1 and 2) discloses a similar technology, wherein a hologram lens with a certain pattern on the surface is utilized to generate the dual focusing effect. The disadvantages are higher cost and lower installation error permissibility (high precision requirement), which decreases the usage of the beam by 15%. PA1 6. Utilize two light sources and two types of lenses. The U.S. Pat. No. 5,777,970 discloses a system using two laser sources with different wavelengths accompanying two lenses of different foci to provide accessing light spots with different sizes. Yet, the drawbacks are higher cost and complicated structure. PA1 7. Use the design of a variable aperture along with a movable objective lens to achieve the goal of dual focusing, as disclosed in the U.S. Pat. Nos. 5,659,533 and 5,281,797. PA1 a first laser beam generator and a second laser beam generator for generating two laser beams with different wavelengths to access the optical storage media; PA1 an optical element, which is a reflective film formed on the surface of a three-beam grating, for controlling the numerical aperture of the second laser diode; PA1 a first beam splitter cube and a second beam splitter cube for separating the two laser beams with different wavelengths from the reflected beam from the optical storage media; PA1 a collimator lens for making each of the two beams of different wavelengths into parallel beams; PA1 an objective lens for focusing each of the parallel beams of two different wavelengths into a spot on the data storage surface of the optical storage media; PA1 a photo detector for converting the reflected beam from the optical storage media into electrical signals; and PA1 a focusing lens for focusing the reflected beam from the first beam splitter cube on the photo detector.